For many years effort has been made to develop low thermal expansion ceramic materials for various applications, such as structural materials, cooking ware, space-craft, substrates for optical mirrors, etc. Materials like .beta.-spodumene, cordierite, .beta.-eucryptite, vitreous silica, borosilicates and other materials have been used for low thermal expansion applications. In a 1984 review article, F. A. Hummel has summarized most of the low/ultra-low thermal expansion materials [Interceram, 33 (6), pgs. 27-30].
References generally regard high expansion materials as having .alpha.&gt;8.times.10.sup.-6, intermediate expansion materials as having 2.times.10.sup.-6 &lt;.alpha.8.times.10.sup.-6, and low or negative expansion materials as having .alpha.&lt;2.times.10.sup.-6. For example, high expansion materials include BeO, MgO, Al.sub.2 O.sub.3 (corundum) and stabilized zirconia. Intermediate expansion materials include SnO.sub.2, SiC, Si.sub.3 N.sub.4, mullite, zircon, ZrTiO.sub.4. And low expansion materials include fused silica, Nb.sub.2 O.sub.5, Ta.sub.2 WO.sub.8, aluminum titanate and cordierite.
Recently, investigation has taken place in the Na.sub.1 +x Zr.sub.2 P.sub.3-x Si.sub.x O.sub.12 and the NaZr.sub.2 P.sub.3 O.sub.12 (or Sljukic, et al. were the first to synthesize NZP-type materials (Preparation and Crystallographic Data of Phosphates with Common Formula M.sup.I M.sup.IV (PO.sub.4).sub.3 ; M.sub.I =Li, Na, K, Rb, Cs; M.sup.IV =Zr, Hf; Croatia Chemica Acta, 39, pgs. 145-148, 1967). They grew single crystals of M.sup.I M.sub.2 (PO.sub.4).sub.3 (M.sup.I =Na, Li, K, Rb, Cs; M.dbd.Zr, Hf) by heating a mixture of alkali metal phosphate and tetravalent metal oxide. The crystal structure of NZP family of materials consists of three-dimensional hexagonal skeleton network of PO.sub.4 tetrahedra sharing corners with ZrO.sub.6 octahedra. Each ZrO.sub.6 octahedron is connected to six PO.sub.4 tetrahedra, while each tetrahedron is linked to four octahedra. The basic unit of the network consists of two octahedra and three tetrahedra corresponding to (Zr.sub.2 P.sub.3 O.sub.12).sup.- ; these units in turn are so connected as to form ribbons along the c-axis, which are joined together perpendicular to the c-axis by PO.sub.4 tetrahedra to develop three-dimensional rigid network. The articulation of these ribbons and chains creates structural holes or interstitial vacant sites in the structure which are normally occupied by Na and/or other substituting ions. There are in fact four such interstitial sites per formula unit of which some are empty depending upon the particular substituion/charge compensation scheme.
The most important and extraordinary feature of NZP structure is its exceptional flexibility towards ionic substitution at various lattice sites. This is due to the strong bonds between Zr--O and P--O creating strong polyhedra. The PO.sub.4 tetrahedra and ZrO.sub.6 octahedra share corners which build up a flexible but stable skeleton network. Na atoms fill in octahedral holes. The chains or ribbons along (001) direction are packed hexagonally. The [Zr.sub.2 (PO.sub.4).sub.3).sup.1- skeleton creates three important structural "holes", the octahedral one (at three fold inversion axis) normally occupied by Na, the prismatic one formed by the phosphate tetrahedra which is normally vacant, and three more octahedral ones (at the center of symmetry), that set up a three-dimensional network.
Alamo and Roy have described the ionic substitution in detail in Crystal Chemistry of the NaZr.sub.2 --(PO.sub.4).sub.3, NZP or CTP Structure Family J. Mat. Sci. 21, pgs. 444-450 (1986). The standard structural formula for the structure is described as follows: M'.sub.1.sup.VI M".sub.3 .sup.X A.sub.2.sup.VI B.sub.3.sup.IV O.sub.12. Sodium goes into M' sites and the M" sites are normally vacant. The following lists the different elements which are substituted at different sites:
M'--Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, H, NG.sub.4, Cu, etc. PA1 M"--Na, K PA1 A--Sn, Ge, Ti, Zr, Hf, Al, Cr, Nb, Ta, rare earths, Fe, Sc, V, etc. PA1 B--P, Si, Al, S, etc.
Substitution can be complete or partial, leading to crystalline solutions of intermediate composition.
An article by Agrawal and Stubican in Material Research Bulletin, v. 20, pages 99-106 (1985) discusses the sintering of Ca.sub.0.5 Zr.sub.2 P.sub.3 O.sub.12. Oota and Yamai characterize the expansions of pure NZP materials in J. Am. Cer. Soc., v. 69 (1), pages 1-6 (1986).